An edge trim is coextruded on a polyolefin film layer. The edge trim protects the film from damage during a stretching operation, wherein clamps, clips or hooks of a tenter frame are attached to the edge regions of the film and the film is stretched in the transverse direction (TD).
Films can be oriented in the machine direction (MD) by passing the film over a series of rollers, whereby a stretching force is applied in the machine direction (MD). Such MD stretching does not require the use of clamps to secure the film.
Tenter frames are often used to stretch films in the transverse direction (TD). In order to achieve TD stretching, it is known to use a film stretching machine including tenter clamps, clips or hooks that are mounted and conveyed on tenter chains and that are guided along expanding guide rails. The tenter clamps hold the edges of the film as it is being stretched.
The use of tenter frames to stretch films is described in U.S. Pat. No. 5,933,930.
When stretching forces become too great, the film will tend to break or tear at points of weakness. A particular point of weakness is where the film is secured by tenter clamps, clips or hooks during TD stretching.
When certain additives are included an extruded film, the ability of the film to resist breaking or tearing during stretching may be reduced. An example of such an additive is a cavitating agent.
Non-cavitated skin layers applied to both surfaces of a cavitated film may help the overall film structure to resist breaking or tearing during stretching. However, it is extremely difficult to make a cavitated film with no skin, especially when the film is stretched in the TD at commercial scale rates and conditions. The tender clips can break the cavitated cells during stretching, which normally takes place in an oven. Such breaking interrupts the stretching process and often results in equipment contamination.
There is provided a biaxially oriented film comprising an extruded polyolefin layer and a coextruded polyolefin edge trim, wherein said edge trim is aligned on both sides of the extruded polyolefin layer in a direction parallel to the machine direction (MD) of the film.
There is also provided a method for making a biaxially oriented film, said method comprising the steps of:
(a) coextruding a polyolefin film layer with polyolefin edge trims on both sides of the extruded polyolefin layer;
(b) stretching the extruded film of step (a) in the machine direction;
(c) attaching clamps, clips or hooks of a tenter frame to the edge trim regions of the film of step (b); and
(d) stretching the film of step (c) in the transverse direction by means of said tenter frame.
The edge trim strengthens the edge regions of the film and provides resistance to breakage in the regions where the film is secured by tenter clamps, clips or hooks.